Sponsored content allows brands to partner with creators to reach creators’ audiences on digital platforms. However, both creators’ and brands’ incomplete understanding of this object generates two critical ambiguities: how to determine the value of sponsored content and how to effectively co-produce it. To better understand these ambiguities, we theorize sponsored content as an epistemic market object: an object that facilitates marketing functions but is only partially understood by the actors who use it . We analyze a data set of interviews, podcasts, media articles, and third-party platform reviews about—and by—content creators, brands, and intermediaries. Our findings show that brands, creators, and intermediaries create and apply knowledge to address valuation and co-production ambiguities. However, this knowledge work is incomplete, creating asymmetries in value outcomes and power relationships in a brand-creator partnership. Our paper contributes to marketing literature and practice by highlighting the role of epistemic market objects in transformative market disruptions that alter the roles of, and the relationships between, market actors. Our findings are transferable to other substantive areas such as Generative AI, Metaverse, NFTs, online news, and the sharing economy.